It sounds like Tree-style Tabs will stop working then as well. That will be a sad day. Tree-style Tabs and some form of mouse gestures are pretty much all I need.
These in particular make Firefox my own (your Firefox is not my firefox, but your firefox is OK), and I will be sad if they disappear without replacement:
Disable Style Button https://addons.mozilla.org/en-US/firefox/addon/disable-style...
Disconnect https://addons.mozilla.org/en-US/firefox/addon/disconnect/
Flash Control https://addons.mozilla.org/en-US/firefox/addon/flash-control...
Ghostery https://addons.mozilla.org/en-US/firefox/addon/ghostery/
It's All Text https://addons.mozilla.org/en-US/firefox/addon/its-all-text/
LastPass https://addons.mozilla.org/en-US/firefox/addon/lastpass-pass...
One Tab https://addons.mozilla.org/en-US/firefox/addon/onetab/
Pinboard https://addons.mozilla.org/en-US/firefox/addon/pinboard-exte...
Privacy Badger https://addons.mozilla.org/en-US/firefox/addon/privacy-badge...
Suspend Tab https://addons.mozilla.org/en-US/firefox/addon/suspend-tab/
uBlock Origin https://addons.mozilla.org/en-US/firefox/addon/ublock-origin...
Uppity https://addons.mozilla.org/en-US/firefox/addon/uppity/
VimFX https://addons.mozilla.org/en-US/firefox/addon/vimfx/
I have a few others which aren't much important to me, but these make FF my own.
This is going to kill the addon community but I guess they don't care at all.
I do know that ones like ghostery and lastpass have financial incentives, while others (like vimfx) are already planning on it. Others don't seem to rely on many (if at all) XUL/XPCOM features